Synthesis and hydrolysis of [alkenyl(alkoxy)carbene]manganese complexes: Evidence for a transient allylic intermediate on the way to alpha,beta-unsaturated aldehydes

A variety of alkenylcarbene complexes [Cp'(CO)(2)MN = C(OEt)CH=CHR] (3) (Cp ' = eta(5)-MeC5H4) was obtained in a staightforward manner upon aldol conde nsation of [Cp'(CO)(2)MN = C(OEt)CH3] (1) with aromatic and alpha,beta-unsa turated aldehydes RC(H)O (2). The reaction is totally stereoselective, givi ng (E)- or (all-E)-alkenylcarbenes only. The protonation of 3 at low temper ature followed by reaction with water affords the alpha,beta-unsaturated al dehyde complexes [Cp'(CO)(2)Mn(eta(2)-RCH=CHCHO] (5), from which the aldehy des RC(H)=C(H)C(H)O (6) were displaced by acetonitrile. The intermediate al dehyde complexes are shown to result from the hydrolysis of a transient cat ionic pi-allyl species [Cp' (CO)(2)Mn(eta 3-RCHCHC(OEt)H](+) ([4](+)) forme d upon protonation of 3.